Various types of microphones and receivers have been used through the years. In these devices, different electrical components are housed together within a housing or assembly. Other types of acoustic devices may include other types of components. These devices may be used in hearing instruments such as hearing aids or in other electronic devices such as cellular phones and computers.
The receiver motor typically includes a coil, a yoke, an armature, and magnets. An electrical signal applied to the coil creates a magnetic field within the motor which causes the armature to move. The armature and reed form a magnetic circuit. While this magnetic circuit is effective at keeping most of the magnetic field within the motor, some portion of this field will leak out of the assembly, and is radiated into the space around it. At radio frequencies, eddy currents prevent the magnetic field from traveling through the magnetic circuit, increasing the radiated field. At these frequencies, the radiation pattern is solely due to the coil.
Hearing instruments typically use a high frequency Pulse Width Modulated (PWM) or Pulse Density Modulation (PDM) signal to power the receivers. These switched signals allow a digital signal processor (DSP) to directly drive the receiver, avoiding the complexity and power inefficiency of performing a digital to analog conversion of the signal before powering the receiver using a traditional analog amplifier. For high efficiency, these systems often use a high carrier frequency, for instance, from 0.2 to 2 MHz. The frequency range just above the carrier frequency of the switching amplifier is a preferred frequency range for radio communications, with the data signal carrier frequency placed at a non-integer multiple of the audio signal carrier to minimize interference.
Many hearing instruments transmit data to other hearing instruments. Unfortunately, the modulation products of the audio signal contain significant energy for another decade above the carrier frequency, overlapping the frequency range of the data signal to be transmitted. In other words, the receiver radiates radio frequency (RF) energy and this interferes with the reception of the transmitted data signals. This, in turn, limits the transmission range of the data signal, and/or requires a higher power signal for transmission. Although higher powered signals can be used, these higher-powered signals create a higher drain on the battery of the acoustic device.
The receiver housing typically is in two pieces, a cup and a cover. The receiver housing is typically made of material such as mu metal which forms a shield to help prevent the magnetic field from escaping. However, the effectiveness of mu metal decreases with frequency. The effectiveness of the shielding at radio frequencies would be improved if a highly conductive material, (e.g. silver, gold, copper, aluminum) were used instead. These highly conductive materials encourage the flow of eddy currents, which will form a magnetic field that is equal and opposite to the field radiated by the receiver motor, hence preventing the field from leaking from the motor. However, these materials have problems such as increased cost, lack of strength, or the shielding of audio frequencies.
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